i have a 1991 w124 300D and the tachometer broke. i found one on ebay and when i installed it, the needle goes 0 or the off position all the way to the max or 70. when i press the accelerator it goes back to the 0 and then flips back to the 70. basically it flips from one end to the other. anyone have any ideas?

Make sure the tach you got is the same as the one you pulled out. On the back side should be a sticker giving the pulse rate the unit is calibrated for. If you have a different tach than original, it won't work properly.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

Is there a red band on the high end of the tach?

Do AC and cruise work?

Sixto
83 300SD
98 E320 wagon

The tach for the 603 diesel is set up to receive 144 pulses per revolution from the ring gear sensor. The gas tach is set up to receive 3 pulses per revolution from the ignition system (since the 1s generation gas 124 300Es in the US all had M103 engines that are 6 cylinder).

If you try to put a gas tach in a diesel it will pin out at 7k as soon as you start the motor.

If the engine is turning 1000 rpm, the diesel tach lead is running at 2400 Hz (144 pulses/rev * 1000 rev/min * 1 min/60 seconds). The gas tach is expecting this to be 50 Hz (3 pulses/rev * 1000 rev/min * 1 min/60 seconds). That is a 48:1 difference, so the gas tach is trying to indicate 48,000 rpm when the diesel engine is running at 1000 rpm.

That is also why a Klima from a gas engine won't work on a diesel.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

The story is this , if you remove a part . Then only replace it with one of the same serial ,or code number as the part you have remove . Also some parts are fitted in the car before you purchase them by other owners , so you dont know if its right to start with .

As previously mentioned, a gas tach won't work on a diesel. I recently went though the same issue on mine. I found a failed electrolytic capacitor burned out on the tach circuit board and a toasted resistor. Replaced both and its good as new.

New blue 51ohm resistor on the left, black 100v 10uF capacitor on the right. The old one was short circuited internally.

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yes it has a red band and my original didn't. the part number is different by one number. so that must be my problem. my cruise and ac work just fine so it isn't the fly wheel sensor, already dealt with that issue few months back. and yes tjts1 that is what happened to me!! i replaced the resistor but not the capacitor so it didn't work. i didn't know the capacitor was blown also. i guess now that i would have to wait on a new tach i could go ahead and try ordering a capacitor instead. i will report back hopefully with good news next week sometime.